This invention relates to a fail-safe device for temperature sensors, and more particularly to a device of this type for temperature sensors used for electronically controlling an internal combustion engine.
Various sensors used for controlling an internal combustion engine include a sensor for detecting the temperature of the engine such as an engine coolant temperature sensor. In general, the engine coolant temperature is detected as representing the engine temperature, and used for various engine control such as fuel injection control. An engine coolant temperature sensor for such applications is required to always generate an output indicative of accurate engine coolant temperature for proper control of the engine. Therefore, if a failure occurs in the sensor, it should be positively detected to thereby take a fail-safe action.
A fail-safe device is known, which detects a failure in the coolant temperature sensor and effects a fail-safe action upon detection of the failure. The device uses a thermistor to detect a failure in the sensor by detecting a terminal voltage of the thermistor, and upon detection of the failure it effects such a fail-safe action as replacement of the actual output value of the sensor by a substitutive value. More specifically, the output value of the sensor is compared with predetermined upper and lower limit values, and if it does not fall within a range between the upper and lower limit values, the sensor is judged to be abnormal, and then the sensor output is replaced by a predetermined substitutive value upon judgement of an abnormality in the sensor.
However, in the above conventional device, the upper and lower limit values are provided for detecting complete disconnection or complete short-circuit in the sensor, so that incomplete disconnection or incomplete short-circuit, i.e. disconnection or short-circuit in which there is some resistance between the power source or the ground and the point at which disconnection or short-circuit occurs cannot be detected, if it exists. Specifically, in the conventional device, the sensor is judged to be abnormal basically when the sensor output value falls out of the range within which it actually can fall when the sensor is normally functioning (for example, the output value becomes equal to the power source voltage upon occurrence of disconnection, while it becomes 0 upon occurrence of short-circuit). Therefore, although complete disconnection or complete short-circuit can be detected, if incomplete failure as mentioned above has occurred and hence the output value falls within the range within which it can fall when the sensor is normally functioning, the sensor is determined to be normal in spite of occurrence of incomplete failure.
If such incomplete failure cannot be detected, the control system can malfunction, failing to carry out accurate control of the engine. That is, in the case that incomplete failure occurs in the sensor, although the sensor output value falls within the range within which it can fall when the sensor is normally functioning, it does not correspond to the actual coolant temperature. The conventional fail-safe device cannot determine the sensor in such an incompletely faulty condition to be abnormal. As a result, the sensor gives, for example, an output indicative of a low coolant temperature even when the actual coolant temperature is high, so that the control system erroneously judges that the engine is in a low temperature condition, and hence controls the fuel injection amount to a value to be applied at the low engine temperature, i.e. the injection amount is erroneously increased.